1. Field of the Invention
The present invention relates to improved methods of connecting optical fibers. More particularly, the present invention relates to methods of connecting optical fibers with each other or with optical waveguides. The present invention also relates to modules of optical waveguides connected to optical fibers and connection aids comprising an array of two or more optical fibers.
The present invention is useful in the field of fiber optics such as optical communication, optical transmission and the like.
2. Description of the Related Art
Optical fibers comprise a core and a cladding surrounding the core, and the core and cladding are both made from quartz, glass and plastic materials. The fibers generally have an outer diameter in the range of about 100 to 300 .mu.m, and the diameter of the core is about 5 to 100 .mu.m. The fibers have many advantages such as low loss, wide bandwidth or high capacity of signal transmission, small diameter, low weight, low cost and flexibility, and because of these advantages, the optical fibers are widely used in various optical devices.
The connection or splicing of optical fibers is usually made as illustrated in FIGS. 1 and 2. Before splicing, a vinyl or other coating or covering 2 is removed from a top portion of each optical fiber 1, and a top face of each fiber 1 is then polished and cleaned. The top faces of the fibers 1 are then placed in contact with each other and adhered with an adhesive 3. In this connection method, however, connection defects such as a dislocation of the optical axis, an inclined optical axis, and a separation of the top faces of the fibers tend to occur. In addition, a satisfactory bonding strength between the fibers can not be obtained and, therefore, the connected fibers must be handled with care.
Similar drawbacks are also found when connecting optical fibers with optical waveguides. Among the prior art methods, two methods for stably connecting the optical fibers are known. For example, K. H. Cameron teaches in Electron. Lett., Vol. 20, pp 974-976, Nov. 1984 that a small block of lithium niobate (LiNbO.sub.3) is attached to an upper surface of an LiNbO.sub.3 waveguide substrate, at the edge of the substrate, so that the end faces of the block and the waveguide formed in the substrate are on the same plane. As is apparent from FIGS. 3 and 4, an LiNbO.sub.3 waveguide substrate 4 has a waveguide 5 formed on the upper surface thereof, which waveguide 5 is produced by a thermal diffusion of titanium, and the end face of the LiNbO.sub.3 substrate 4 and that of the LiNbO.sub.3 block 6 are on the same plane. An optical fiber 1 consists of core 7 and cladding 8, and this cladding 8 is removed from the top portion of the fiber 1. The core 7 of the fiber 1 is aligned with an end face of the Ti:LiNbO.sub.3 waveguide 5 and while maintaining an intimate contact between the core 7 and the waveguide 5, a suitable adhesive 3 is applied to the surrounding contact area. This connection method is simple and provides a low excess loss because only the end face of the fiber is in contact with the waveguide, but the resulting connection is mechanically weak. Further, since a large amount of the adhesive must be used for this connection, dislocation of the axis of the fiber and separation of the top face of the fiber from the waveguide frequently occur as a result of an expansion of the adhesive in an elevated environmental temperature.
Another method is to secure the optical fiber in a V-grooved silicon block, as taught by E. J. Murphy et al., IEEE J. Lightwave Technol., Vol. LT-3, pp 795 -799, Aug. 1985. As illustrated in FIGS. 5 and 6, two V-grooved silicon blocks 9 and 10 are combined and an optical fiber 1 from which a coating 2 is removed is secured in a V-groove 11 in the combined blocks 9 and 10. As in the Cameron method, an LiNbO.sub.3 substrate 4 with a Ti:LiNbO.sub.3 waveguide 5 is provided with an LiNbO.sub.3 block 6. Since the substrate 4 and the block 6 have the same plane at the end face, they are firmly bonded with an end face of the blocks 9 and 10 by the action of an adhesive 3 applied therebetween. According to this method, a satisfactory bonding strength between the fiber 1 and the waveguide 5 can be attained, because the bonding is made between the large area of the waveguide substrate 4 and block 6 and the large area of the blocks 9 and 10, to secure the fiber 1. However, this method suffers from drawbacks in that, for example, many production steps are needed to complete this method, and thus it is cumbersome and expensive. Further, in order to avoid excess loss, it is essential to precisely align the fiber and waveguide. In practice, often a large excess loss occurs because of an imperfect matching between the end face of the fiber and that of the waveguide. Assuming that the end face of the fiber is disposed at an inclined angle of 1.degree. from the end face of the waveguide, a gap of several tens of micrometers will be produced between the fiber and waveguide, and this gap results in large excess loss. Similar undesirable results will be induced when the end surface of the fiber or waveguide is polished at an angle of 1.degree. from the correct polishing plane. It is therefore desirable to provide an improved low-loss, high-strength connection method for optical fibers and/or optical waveguides.
The connection between the optical fibers and optical waveguide can be applied to the production of optical waveguide modules which can be advantageously used in optical communication and optical sensing, for example. Two typical examples of the prior art waveguide modules are shown as a plane view and a cross-sectional view in FIGS. 7 and 8.
In FIGS. 7(A) and 7(B), the optical waveguide module comprises a box or housing 13. Optical fibers with the coating 2 are secured to the wall of the box 13. An exposed fiber is placed in contact with and connected with an end face of the waveguide 5 on the substrate 4. This connection is carried out by inserting the exposed fibers into V-shaped grooves of the fiber holder 12 of silicon, fixing the fibers with an adhesive, and covering an upper surface of the holder 12 with a glass plate or similar parts (not shown). After polishing the end faces, the end faces of the fibers are aligned with the corresponding end faces of the waveguides and are then bonded with an adhesive.
FIGS. 8(A) and 8(B) illustrate a modification of the optical waveguide module of FIGS. 7(A) and 7(B). Here, the wall of the box 13 is used to secure the fiber holder 12, instead of securing the coated fibers.
These prior art waveguide modules, however, have problems to be solved, which problems arise when the box containing the waveguide substrate and securing optical fibers or fiber holders is subjected to an influence of external force or heat. When the box is deformed or distorted by an external force or expanded by heat, such undesirable changes in the box can directly affect the connection area of the waveguides and fibers. Namely, since a relative position of the end face of the fiber with regard to the end face of the waveguide is slightly shifted, the light coupling efficiency of the fiber to the waveguide is reduced, and in the worst case, defects such as a separation of the end face of the fiber from that of the waveguide and the like occur.
In the production of optical devices such as optical waveguide modules, optical fibers are generally not used alone but in combination as an array. This is because two or more optical fibers are necessary to satisfy the requirements concerning an increased amount of information to be transmitted. The fiber holders having an array of two or more optical fibers secured therein have been produced in accordance with the manner previously described with reference to the production of the waveguide modules and the manner described hereinafter with reference to FIGS. 9(A) to 9(D), for example.
The production of the fiber holder will be now described with reference to FIGS. 9(A) to 9(D): At this time, silicon (Si) substrates are conventionally used as a material of the fiber holder because they can be anisotropically etched to form V-shaped grooves on the surface thereof. Namely, as is well known in the field of semiconductor devices, when the Si single cystals are etched with a heated alkali etchant, anisotropical etching is attained because they show different etching rates depending upon the direction of crystalline faces thereof. For example, when the Si single crystal is etched in the above manner, (100) and (110) faces of the crystal are rapidly etched, but the crystal having (111) face is etched very slowly. Based on these characteristics of the Si single crystals, V-grooved Si substrates can be easily produced.
First, as shown in FIG. 9(A), parallel V-grooves 15 are formed on an upper surface of the Si substrate 14. While not illustrated herein, a wafer of Si single crystal having a specific crystal structure is prepared and a mask pattern of silicon oxide (SiO.sub.2) coating is formed on the upper surface of the Si wafer. The mask pattern covers areas other than those in which the V-grooves are etched in the subsequent step. The masked wafer is then dipped in an alkali etchant to form a plurality of parallel fine V-grooves. After removal of the SiO.sub.2 mask, the illustrated Si substrate 14 with parallel V-grooves 15 can be obtained.
Second, as shown in FIG. 9(B), optical fibers 1 from which the protective vinyl coating has been removed are placed in each of the V-grooves 15 of the substrate 14 and an epoxy-type adhesive 16 is coated to fill the gaps between the V-grooves 15 and fibers 1 and to cover the fibers 1 on the substrate 14.
After application of the adhesive, the fibers-containing substrate 14 is covered with another V-grooved Si substrate 17 as in FIG. 9(C), and the adhesive then allowed to harden. In this stage, as illustrated, the fibers 1 protruding from an end face of the bonded substrates 14 and 17 may have different lengths.
Next, the substrates 14 and 17 are laterally cut at a predetermined distance from the end face thereof (see lines b-b' of FIG. 9(C)). This cutting is made in a direction perpendicular to the fibers 1 in the V-grooves 15. The newly exposed end face of the substrates 14 and 17 is then polished to obtain the fiber holder shown in FIG. 9(D).
The resulting fiber holder with fiber array is shown in a longitudinal cross section in FIG. 10. The optical fiber 1 consists of a core 7 and a cladding 8 and is bonded through a coating of the adhesive 16 to the Si substrates 14 and 17. An end face of the fiber holder indicated with an arrow (a) has a planar surface.
The fiber holder of FIG. 10 can be attached to a waveguide as shown in FIG. 11. The waveguide used herein consists of an LiNbO.sub.3 substrate 4 with a Ti:LiNbO.sub.3 waveguide 5 and an LiNbO.sub.3 block 6. An end face of the waveguide also has a planar surface. After alignment of the position of the core 7 of the fiber 1 with that of the waveguide 5, a UV-hardenable epoxy-type adhesive 18 is coated around the bonding interface between the fiber holder and the waveguide. Finally, UV rays are irradiated to harden the adhesive 18 and complete an integral bonding of the holder and waveguide.
However, there are some drawbacks in this and other prior art fiber holders, one of which drawbacks is an increase of the connection loss occurring as a result of a formation of fine gaps between the core of the fiber and the waveguide when a polished end face of the fiber holder and/or that of the waveguide are inclined or they are bonded with an inclination therebetween. For example, if either the holder or the waveguide has an end face inclined at an angle of 1.degree., a gap of several tens of micrometers will be formed between the core of the fiber and the waveguide. Further, in order to avoid a rapid increase of the connection loss, it is necessary to limit the level of the mismatching between the core of the optical fiber and the waveguide to 0.5 .mu.m and less when both the core and the waveguide have a diameter of about 10 .mu.m. Accordingly, when the fiber holder is attached to the waveguide in accordance with the prior art methods, connection and alignment operations must be carried out using mechanical means such as fine adjustment dials and the like, which make said operations cumbersome and expensive. Therefore, an improved fiber holder containing fibers in array is desired.
As described above, in the prior art, connection of the optical fiber to the waveguide is generally carried out by, as illustrated in FIG. 12, aligning a core 7 of the fiber 1 with a waveguide 5 and securing them with an adhesive 3. For this illustrated instance, assuming that both the waveguide substrate 4 and the block 6 are made from LiNbO.sub.3, the waveguide 5 from Ti-diffused LiNbO.sub.3 and both the core 7 and the cladding 8 from quartz (SiO.sub.2), since there is a large difference in the refractive index between the Ti:LiNbO.sub.3 waveguide 5 and the SiO.sub.2 core 7, light l.sub.1 transmitted through the core 7 is partially reflected at an end face of the waveguide 5 and returned when it enters the waveguide 5. The returning light is indicated by the dotted line l.sub.2. Such partial reflection of the incident light l.sub.1 must be prevented, for the following reasons.
In a typical optical communication system such as that of FIG. 32, for example, the system comprises a DFB-type semiconductor laser 19 and a waveguide-type optical modulator 20 in which the waveguide is made from Ti:LiNbO.sub.3. Laser beams from the laser 19 are transmitted through the SiO.sub.2 fiber 21 to the modulator 20. After modulation, the beams are transmitted through the SiO.sub.2 fiber 22 to the next device (not shown). As in the case of FIG. 12, incident laser beams on the modulator 20 are partially reflected at an interface between the end face of the waveguide of the modulator 20 and that of the fiber 21. The reflected beams then return to and adversely affect the laser 19, thereby causing variations in the characteristics of the laser.
In the prior art system, in order to inhibit this return of the laser beams to the laser, conventionally an isolator is disposed between the laser 19 and the modulator 20. However, if the returning beams are notably increased, it is necessary to use a high quality isolator, and this means an increase of the costs of the resulting optical communication system. Alternatively, in order to reduce the degree of return of the incident laser beams, it was proposed to apply an anti-reflection coating onto an end face of the waveguide. However, this coating technique is not satisfactory in regard to the control of the thickness of the resulting coating, in addition to resulting in an increase of the steps necessary to produce the devices, and accordingly, an increase of the production costs. In view of these disadvantages, it is desired to provide an improved connection method for optical fibers which can prevent or reduce reflection of the beam incident onto the waveguide without complicating the system.